From 75c382fe163df8f8b7feb7245922b1b83835cf30 Mon Sep 17 00:00:00 2001
From: Rusty Russell <rusty@rustcorp.com.au>
Date: Tue, 8 Nov 2022 11:41:44 +1030
Subject: [PATCH] lightningd: --dev-onion-reply-length option.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
 common/sphinx.c      | 11 +++++++++--
 common/sphinx.h      |  3 +++
 lightningd/options.c |  5 +++++
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/common/sphinx.c b/common/sphinx.c
index fff2a2d29..2fef1451f 100644
--- a/common/sphinx.c
+++ b/common/sphinx.c
@@ -679,12 +679,19 @@ struct route_step *process_onionpacket(
 	return step;
 }
 
+#if DEVELOPER
+unsigned dev_onion_reply_length = ONION_REPLY_SIZE;
+#define OUR_ONION_REPLY_SIZE	dev_onion_reply_length
+#else
+#define OUR_ONION_REPLY_SIZE	ONION_REPLY_SIZE
+#endif
+
 struct onionreply *create_onionreply(const tal_t *ctx,
 				     const struct secret *shared_secret,
 				     const u8 *failure_msg)
 {
 	size_t msglen = tal_count(failure_msg);
-	size_t padlen = ONION_REPLY_SIZE - msglen;
+	size_t padlen = OUR_ONION_REPLY_SIZE - msglen;
 	struct onionreply *reply = tal(ctx, struct onionreply);
 	u8 *payload = tal_arr(ctx, u8, 0);
 	struct secret key;
@@ -716,7 +723,7 @@ struct onionreply *create_onionreply(const tal_t *ctx,
 	 *     - Note: this value is 118 bytes longer than the longest
 	 *       currently-defined message.
 	 */
-	assert(tal_count(payload) == ONION_REPLY_SIZE + 4);
+	assert(tal_count(payload) == OUR_ONION_REPLY_SIZE + 4);
 
 	/* BOLT #4:
 	 *
diff --git a/common/sphinx.h b/common/sphinx.h
index 6ba537cf3..9b80c29b3 100644
--- a/common/sphinx.h
+++ b/common/sphinx.h
@@ -267,6 +267,9 @@ sphinx_compressed_onion_deserialize(const tal_t *ctx, const u8 *src);
 #if DEVELOPER
 /* Override to force us to reject valid onion packets */
 extern bool dev_fail_process_onionpacket;
+
+/* Override to set custom onion error lengths. */
+extern unsigned dev_onion_reply_length;
 #endif
 
 #endif /* LIGHTNING_COMMON_SPHINX_H */
diff --git a/lightningd/options.c b/lightningd/options.c
index 91f443643..07504bda9 100644
--- a/lightningd/options.c
+++ b/lightningd/options.c
@@ -752,6 +752,11 @@ static void dev_register_opts(struct lightningd *ld)
 	opt_register_noarg("--dev-no-ping-timer", opt_set_bool,
 			   &ld->dev_no_ping_timer,
 			   "Don't hang up if we don't get a ping response");
+	opt_register_arg("--dev-onion-reply-length",
+			 opt_set_uintval,
+			 opt_show_uintval,
+			 &dev_onion_reply_length,
+			 "Send onion errors of custom length");
 }
 #endif /* DEVELOPER */